Responses of soil microorganisms to volatile exudates from germinating pea seeds

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Abstract
Responses of soil microorganisms to volatile exudates from germinating pea seeds of differing quality were determined. Germination of sclerotia of Rhizoctonia solani and Sclerotium rolfsii and subsequent hyphal growth were stimulated by exposure to volatiles from aged but not nonaged pea seeds. Hyphae grew preferentially toward aged seeds. In natural soil, bacterial and fungal populations showed significant increases after exposure to volatiles from aged seed. For example, Fusarium spp. and Pseudomonas spp. showed increases of 79 and 2200%, respectively, over their original population levels after a 48-h exposure to volatiles. Conversely, Pythium populations and associated seed-rotting potential of soil decreased in natural soils exposed to volatiles. In autoclaved soils infested with P. ultimum (PHP4), Pythium populations increased dramatically after exposure to volatiles from aged pea seeds. In soils infested with either soil fungi or bacteria in addition to P. ultimum, Pythium levels remained constant or decreased, respectively, with time of exposure. Exposure to the volatiles from aged pea seeds stimulated soil microbial activity. These results suggest that Pythium germlings, when unable to reach a host, are subjected to microbial antagonism in the presence of the native soil microflora. A decrease in cucumber seed rot coincided with decreases in Pythium numbers.